(1) Field of the Invention
The present invention is related to a Fourier-transform nuclear gyromagnetic resonance spectrometer, and particularly to a Fourier-transform nuclear gyromagnetic resonance spectrometer which is also capable of measuring the nuclear gyromagnetic resonance of continuous waves.
(2) Description of the Prior Art
An atomic nucleus of which center of gravity is at rest has a given angular momentum which is often called nuclear spin. The nuclear spin of the atomic nucleus is given by hI/2.pi., and the magnetic moment is given by .gamma.h/2.pi.I, where h represents Plank's constant, I the spin quantum number, and .gamma. a constant (gyromagnetic ratio) specific to the nucleus, which is 1/2 for 1.sub.H, 19.sub.F, 13.sub.C, 31.sub.P, and 1 for 14.sub.N. The presence of angular momentum or the accompanying magnetic moment in the atomic nucleus represents that the nucleus is spinning. If the spinning atomic nucleus is placed in a unidirectional magnetic field, the nuclear spin is so polarized that the direction possessed by the spinning axis is restricted to either the direction of the magnetic field or the direction opposite thereto, and the magnetic moment performs the precession about the spinning axis. The energy of the nuclear spin is given by the product of the magnetic moment and the magnetic field, but the nuclear spin is separated into a plurality of energy levels (Zeeman effect) to exhibit Boltzmann's distribution. Therefore, if electromagnetic waves corresponding to the difference between a high energy level and the next high energy level are radiated at right angles to the magnetic field, the electromagnetic waves are absorbed (resonance absorption) causing the nuclear spin to be transited from a low level to a high level. The condition in which the resonance absorption takes place is given by .omega.=.gamma.H, where .omega. represents an angular frequency of the electromagnetic wave and H represents the intensity of the unidirectional magnetic field. According to the above relation, the resonance takes place at the same frequency as far as the nucleus is the same, and no other effective applications are obtained than to detect the above-said nucleus.
The atomic nucleus to be practically measured does not exist naked but is occupying its position in a particular molecule. Therefore, even when dealing with homonuclei, the atomic nuclei exhibit shifting resonance frequencies depending upon whether they are present naked or not, being caused by the magnetic shield established by the electrons encircling the nucleus and by other neibouring nuclear spins. Therefore, the relation .omega..sub.1 =.gamma.H(1-.sigma.)+.DELTA..omega..sub.J is practically applied to the nucleus in the molecule, where .omega..sub.1 denotes a resonance frequency of a naked nucleus, a screeing constant, and .DELTA..omega..sub.J a frequency shift corresponding to a spin-spin coupling energy. The term .gamma.H(1-.sigma.)=.DELTA..omega.=.delta. of this equation is called chemical shift. The symbol .delta., however, is not a shift of the naked nucleus, but denotes a shift found from the resonance of a nucleus of a particular molecule (TMS [tetramethylsilane] in the case of a proton). Therefore, if the screening constant of the nucleus of a reference substance is denoted by .sigma..gamma., the chemical shift is given by .delta.=.epsilon.H(.sigma.-.sigma..gamma.).
The resonance occurs in the region of radio waves, and this serves as an added source of energy. As there develops resonance absorption in the radio waves, the absorbed energy appears in the form of a change in magnetization as a whole nuclear spin. This appears not only in the direction (direction Z) of the unidirectional magnetic field but also appears in the form of a gyrational change in a plane (plane X-Y) at right angles thereto. The change in gyrational magnetization further works to change the impedance of a coil which is magnetically coupled to the nuclear spin to give a high-frequency energy to the nucleus. Accordingly, the change in gyrational magnetization can be measured by an impedance-bridge method (single-coil method) and the like. Another method (cross-coil method) for measuring the change in gyrational magnetization will consist of placing a detector coil which crosses a radiation coil in a plane X-Y, to detect the voltage induced in the detector coil by said change in gyrational magnetization.
One of the principal objects of the nuclear gyromagnetic resonance spectrometer is to measure the aforesaid chemical shift which gives a clue for determining the molecular structures. The nuclear gyromagnetic resonace spectrometers may be classified into those of the type in which either one of the high-frequency wave or the magnetic field is fixed and the other is continuously changed, and those of the type in which the magnetic field is fixed and the high-frequency waves applied to the specimen is pulse-modulated. The former devices, in contrast to the latter devices, are in many cases called continuous wave nuclear gyromagnetic resonance spectrometers, and the latter devices are called Fourier-transform nuclear gyromagnetic resonance spectrometers. With the Fourier-transform nuclear gyromagnetic resonance spectrometer, a plurality of homonuclei are simultaneously resonated to detect the resulting transient resonace signals which are called free induction decay signals. Here, since the transient resonance signals contain resonance frequency components within a frequency range given by an inverse number of a pulse width when the high-frequency is to be pulse-modulated, the transient resonance signals are Fourier-transformed to obtain nuclear gyromagnetic resonance spectrum within the range of said frequencies. When used in combination with a signal accumulating device, the Fourier-transform nuclear gyromagnetic resonance spectrometer makes it possible to obtain the nuclear gyromagnetic resonance spectrum having improved signal to noise ratio within reduced period of time, thus giving advantages superior to the continuous wave nuclear gyromagnetic resonance spectrometers. The Fourier-transform nuclear gyromagnetic resonance spectrometers have, therefore, been increasingly used in recent years.
The nuclear gyromagnetic resonance spectrometers must have high resolving power to measure small chemical shifts and must also have a function to adjust the resolving power such that desired resolving power can be attained. The resolving power can be adjusted by correcting the non-uniformity in the magnetic field. The magnitude of peak and upsurge of the nuclear gyromagnetic resonance spectrum, width, symmetry, expansion of foot of the peak, play important roles for determining the quality of the resolving power. Hence, the resolving power can be accurately and easily adjusted if the non-uniformity of the magnetic field is so corrected that the abovesaid factors fall under optimum conditions while observing the spectrum. With the Fourier-transform nuclear gyromagnetic resonance spectrometer which requires several seconds for performing the Fourier-transform, however, it is virtually very difficult to correct the non-uniformity of the magnetic field in order to adjust the resolving power while observing the display of the spectrum. With the Fourier-transform nuclear gyromagnetic resonance spectrometer, therefore, the resolving power is often adjusted by observing the display of free induction decay signal waveforms before being Fourier-transformed. In practice, however, the display of free induction decay signal waveforms merely yields information which is related to the magnitude of the peak of spectrum, from which it is virtually impossible to accurately adjust the resolving power.